Pursuant to 35 U.S.C. xc2xa7119(a)-(d), this application claims priority from Taiwanese application no. 91104506, filed on Mar. 11, 2002.
1. Field of the Invention
The present invention relates to a laser driver, and especially to a high-speed laser driver including wave shaping and dynamic control circuits, which uses a source degeneration resistor instead of a prior constant current source to increase the operating speed, a buffer with a dynamic loading source resistor to buffer the output differential signals to the next circuit, and a tunable device to control output gain and prevent overshoot.
2. Description of Related Art
FIG. 1 is a block diagram of a typical laser driver 10. As shown in FIG. 1, the laser circuit 10 includes a wave-shaping circuit 11 to receive and shape signals Sdp and Sdn externally, an output control circuit 12 to adjust signals Sdp and Sdn to desired signals Swu and Swl, and an output stage circuit 13 to convert signals Swu and Swl into a desired output I for the laser diode 14 by externally inputting the signal CG to set the desired modulation current. As such, the laser circuit 10 drives the modulation current Im of a laser diode 14. When the laser diode 14 functions, the required bias voltage is provided by a bias current source 15. However, for high-speed and broadband (NRZ signal) requirements of laser drivers, such a conversional MOS constant current source differential pair structure cannot meet the design specification because the speed limitation comes from the parasitic capacitance and some physical behavior of MOS devices. Thus, the laser drive with the conversional structure has poor performance. For example, FIG. 2 is an example of the wave-shaping circuit 11 of FIG. 1. As shown in FIG. 2, the wave-shaping circuit 11 has constant current source differential pairs. These constant current source differential pairs serve as gain stages and limit the output swing to a desired level. For simplification, only the last differential stage 111 consisting of resistors R1, R2, MOSs M1, M2, and a constant current source CS1 is shown. Further, circuit 112 serves as an output stage to drive the next circuit and adjusts the output voltage level to a desired swing. A wave-shaping circuit like 11 can convert a differential sine wave pair Sdp, Sdn into a differential square wave pair Swu, Swl or a single-end signal into a differential pair (not shown). However, as mentioned above, the operating speed of a constant current source differential pair is limited by the non-ideal characteristics and of MOS current source. Thus, as shown in FIG. 3, a parasitic capacitance Cp is formed due to the junction voltage difference Vdxe2x88x92Vb between the drain D and bulk B of the MOS, for example M1 (FIG. 2) when an external current Iext is input to the drain D of M1. The parasitic capacitance Cp causes the current source CS1, CS2 and CS3 in FIG. 2 to be non-ideal current sources such that the operating speed of the differential pair M1, M2 is limited. When operating at high speed, the total current of NMOSs M1 and M2 is equal to the current of CS1 in ideal. However, the parasitic capacitance Cp in the non-ideal constant current sources can cause signal distortion. For example, when a 0.35 xcexcm CMOS manufacturing process is used to shape a 1.25G clock pulse, as shown in FIG. 4, the signal such as Swu may change the waveform from V1 to V2 and cause a signal distortion. If the signals Swu, Swl are unchanged, and sent to the next output control circuit 12, as shown in FIG. 5, the circuit 12 also includes MOSs M51-53, causing output signal distortion. That is, the MOS M53 becomes a non-ideal constant current source and the signals Sout1, Sout2 sent to the next output stage circuit 13 (FIG. 1) change the waveform from V1 to V2 (FIG. 4) so as to cause output signal distortion. Likewise, the waveform distortion may present in the output stage circuit 13 consisting of, for example, a resistor R53, MOSs M54-M56 as mentioned above.
Accordingly, an object of the invention is to provide a high-speed laser driver including a wave-shaping circuit, which performs wave shaping for the laser driver at high speeds.
Another object of the invention is to provide a high-speed laser driver including wave-shaping and dynamic control circuits, which performs the wave shaping and the dynamic control for the laser driver at high speed.
The invention provides a high-speed laser driver including wave-shaping circuits, which performs wave shaping for driving the laser at high speed. The laser driver includes a wave-shaping circuit having a source degeneration resistor instead of a prior constant current source to increase the operating speed and a buffer with a dynamic loading source resistor to buffer the output differential signals to the next circuit. The laser driver further includes a dynamic control circuit having a tunable device to control output gain and prevent overshoot.